More than 40 percent of children now are born to women who aren't married. As coauthors of "Red Families v. Blue Families," we often give talks about the recent rise in what's called the "nonmarital birth rate." Sometimes someone will come up to us, confess his or her encounter with single parenthood, and say something like: "When my daughter got pregnant and decided to keep the child, we were OK with that because we are Christians. When she decided not to marry the father, we were relieved because we knew he would be bad for her and the marriage would never work."

They express these two beliefs -- that they are Christian and thus uncomfortable with abortion, and that they are relieved their daughter decided to raise the child alone -- as if they are not connected. But in fact this may be one of the stranger, more unexpected legacies of the prolife movement that has arisen in the 40 years since Roe vs. Wade:

In conservative communities, the hardening of antiabortion attitudes may have increased the acceptance of single-parent families. And by contrast, in less-conservative communities, the willingness to accept abortion has helped create more stable families.

Researchers have considered many reasons for the rise in the nonmarital birth rate -- the welfare state, the decline of morals, increasing independence of women, even gay marriage. But one that people on neither the left nor the right talk about much is how it's connected to abortion.

The working class had long dealt with accidental pregnancy through the shotgun marriage. As blue-collar jobs paying a family wage have disappeared, however, so has early marriage. Women are then left with two choices: They can delay childbearing (which might entail getting an abortion at some point) until the right man comes along, or get more comfortable with the idea of becoming single mothers. College-educated elites have endorsed the first option, but everyone else is drifting toward the second.

In regions and social classes where the stigma for having an abortion is high, the nonmarital birth rate is also high. Without really thinking about it or setting up any structures to support it, women in more-conservative communities are raising children alone. This is a legacy the prolife movement has not really grappled with.

In "Red Families v. Blue Families," we pointed out the irony that blue states, despite their relatively progressive politics, have lower divorce and teen birth rates than do red states. In fact, the college-educated middle class, partly by postponing having children, has managed to better embody the traditional ideal: that is, a greater percentage of children being raised in two-parent families. In response, New York Times columnist Ross Douthat admitted that "it isn't just contraception that delays childbearing in liberal states, it's also a matter of how plausible an option abortion seems, both morally and practically ..." Although he conceded that "the 'red family' model can look dysfunctional -- an uneasy mix of rigor and permissiveness, whose ideals don't always match up with the facts of contemporary life," he argued that "it reflects something else as well: an attempt, however compromised, to navigate post-sexual revolution America without relying on abortion."

He's right. The big secret very few are willing to discuss is that abortion rates do seem to correlate with greater commitment to marriage. Although the college-educated have a relatively low number of abortions, a higher percentage of their unplanned pregnancies end in abortion than for any other group. The college-educated almost certainly think of themselves as embracing the pill and resorting to abortion only in the relatively rare cases where contraception fails. Yet the bottom line is that the willingness to abort, however infrequently it occurs, makes it possible to reinforce the norm against having a child outside of marriage.

Sociologist Averil Clarke points out that unmarried white college grads, who have maintained a 2 percent nonmarital birth rate for the last 20 years, terminate a higher percentage of pregnancies than do other groups. And urban theorist Richard Florida finds that the higher a state's abortion rate, the lower its divorce rate.

This creates the dilemma Douthat identified for those who see abortion as immoral. The Christian right preaches that contraception is not perfect, that sex inevitably risks pregnancy and that abstinence provides the only solution. And indeed, as the number of abortions has dropped, the rate of unmarried women giving birth has increased. Nonelite young women often give their opposition to abortion as an explanation for why they went ahead and had the child, even if in other ways religion has not influenced them much.

Think of it as the Bristol Palin effect, after Sarah Palin's daughter, who at age 17 announced her pregnancy shortly after her mother's selection in 2008 as the Republican vice-presidential candidate. Republican women applauded the Palins' choice to support their daughter's decision to have the child. They wrote that the Palins were sticking to their values rather than doing what other Republican leaders in such situations had done (quietly arranging an abortion). Democratic women were appalled -- mystified why anyone thought having a 17-year-old raise a child was a good idea.

Liberal and conservative women did agree on one thing, however: Neither group thought there was any point in having Bristol marry Levi Johnston, the father of the child.

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Naomi Cahn is a law professor at George Washington University. June Carbone is a law professor at the University of Missouri-Kansas City. They are coauthors of "Red Families v. Blue Families and Family Classes," which will be published in 2013 by Oxford University Press.